Use Bundler (again)

Looking at the Git history, there was an attempt to use Bundler in the past which was then reverted. I can't find the details of why or think of any compelling reasons, so let's try this again.

This change makes the development workflow much more sane - just update the Gemfile and `bundle install` to install and develop against the specific gems and versions. This is particularly useful when adding, testing, updating, or removing Gems. We can also be sure that (eg. with `bundle exec`) we're ONLY using the specified set of Gems and corresponding versions.

Bundler is also used by default in Crowbar 2.0 (Rails 3 and above), so this makes the transition easier.

Note that is does _not_ mean we have to use `bundle install` to install Gems. You can do that as a Crowbar developer if you wish, or just stick to the .deb/.rpm packages as you have been doing and in the product.